1. Field of the Invention
The present invention relates to a cost-efficient, accurately-assembled rotor structure (i.e., a rotor) that is supported, in a non-contact state, by a radial self-acting air bearing, and a polygon scanner using the rotor so as to provide enhanced reliability of the polygon scanner as a result of how a rotating member of the polygon scanner is held.
2. Discussion of the Background
Electrophotographic recording apparatuses, such as laser printers or digital copying machines etc., that operate by way of forming a latent image with a light beam are proliferating as a result of their characteristic high image quality, high speed recording, low noise etc., and low price. Such apparatuses include a light source pointed at a rotatable polygon scanner that rotates at 20,000 revolutions per minute (rpm) for high speed recording and high density processing applications. However, if used, conventional bearings that support the rotatable polygon scanner suffer from reliability, noise and longevity (bearing life) concerns as a result of the high speed operation. Therefore, self-acting air bearings that provide support to the scanner are used where the support is provided by a cushion of air.
In reference to FIGS. 19 and 20, Japanese Laid Open Patent No. 5-216707 discloses a self-acting air bearing that supports a high speed rotation of a polygon mirror 2. A rotor 1 is structured in a way so as to mount the polygon mirror 2 on an upper surface of a flange portion 3a that is formed around a center of a shaft direction of a hollow rotating shaft 3 and by holding and fixing the polygon mirror 2 by clamping a mirror keeper 4, which closes one edge of the hollow rotating shaft 3 by a screw, and by engaging a rotor yoke 7 that mounts a rotor magnet 6 in an inner side with a lower surface of the flange portion 3a and by adhering the rotor yoke 7 by way of glue. The rotor magnet 6 and the rotor yoke 7 face a driving coil on a housing side (not shown) and form a driving portion (a motor portion) on a rotor side so that the polygon scanner forms an axial gap direct current brushless motor.
In rotor 1, a fixing shaft that is fixed in a housing is inserted into the hollow portion of the hollow rotating shaft 3. Arranged this way, the rotor 1 is radially supported in a noncontact state by generating a self acting air flow between the rotor 1 and a surface of the hollow rotating shaft 3. Air is conveyed by a groove, such as a herringbone groove, that is formed on a peripheral surface of the fixing shaft. Axially, the rotor 1 is supported in a noncontact state by magnetic force that repels in a vertical direction by facing a permanent magnet 5, mounted on the mirror keeper 4, with permanent magnets that are mounted on an upper portion of the fixing shaft and a cover not shown in FIG. 19. In FIG. 19, a groove 6a is for correcting balance so as to avoid generating large vibrations when the rotor 1 rotates at high speed.
In FIG. 20, with respect to the hollow rotating shaft 3, two states that are viewed from upper diagonal direction and lower diagonal direction are shown, however, the views do not indicate that the hollow rotating shaft 3 needs two pieces. Rather, the views are shown this way so that mounting portions of an upper side and a lower side of the hollow rotating shaft 3 are easy to be understood, and preferred embodiments that will be described below will be presented in a similar fashion.
As a driving portion that rotates the rotor, other than axial gap type rotors, for instance, there are direct current brushless motors of a radial gap-inner rotor type, as disclosed in Japanese Laid Open Patent No. 5-231427. Also, radial gap outer rotors are available, as disclosed in Japanese Laid Open Patent No. 61-9138.
However, such conventional self acting air bearing type polygon scanners are subject to getting hot during high speed rotation and thus subjected to repeated temperature swings as the polygon scanner is cycled on and off during its life span. As a consequence, the glue used for fixing a rotor yoke 7 to the lower surface of the flange portion 3a of the hollow rotating shaft 3 is reduced in strength. The glue is also subject to shearing stress do to differences in coefficient of thermal expansion of the hollow rotating shaft 3 and the rotor yoke 7. Subject to such stains, the glue eventually fails and the rotor yoke 7 shifts, causing the rotor 1 to become unbalanced. Further, in an extreme case, there is a problem that the rotor yoke 7 falls from the hollow rotating shaft 3.
The mirror keeper 4 of the rotor 1 clamps and fixes the polygon mirror 2 that is mounted on the upper surface of the flange portion 3a of the hollow rotating shaft 3 by passing the screw 8 through the through hole 4a and clamping a screw hole 3b of the hollow rotating shaft 3 with a screw 8. Thus, the processes for forming the through hole 4a in the mirror keeper 4 and for forming the screw hole 3b in the hollow rotating shaft 3 must be precise, which increases expense. Further, residual cutting oil may remain within the screw hole 3b, where the cutting oil scatters during high speed rotation and soils optical parts such as the polygon mirror or a window glass that is mounted in the cover etc., and therefore elaborate cleaning is required, with associated higher expense.
Further, in the self acting air bearing type polygon scanner, surface accuracy of the reflecting surface of the polygon mirror 2 is required. Since a mounting surface of the flange portion 3a is processed coaxially and unitarily with the hollow rotating shaft 3, it has no problem. However, an edge surface of the mirror keeper 4, for pressing the upper surface of the polygon mirror 2, requires high processing accuracy and assembling accuracy and thus also increases the cost.